1. Field of the Invention
The present invention relates to a diamond cutting tool, and more particularly, it relates to a cutting tool having a cutting edge of diamond formed by vapor deposition that is excellent in wear resistance, heat resistance and chipping resistance. The invention further relates to a method of manufacturing such a cutting tool.
2. Background Information
Diamond, which is excellent in hardness and thermal conductivity, is particularly useful as a material for forming a tool for cutting otherwise uncuttable materials. Such diamond materials applicable to tools are classified into single-crystalline and polycrystalline diamond materials. However, single-crystalline diamond is extremely high-priced, hard to work and easy to cleave, although the same is an excellent material in consideration of its physical characteristics.
On the other hand, polycrystalline diamond materials which are applicable to tools can be roughly classified into two types. The first type is sintered diamond, which is obtained by sintering fine diamond powder and an iron family metal such as Co under diamond-stable, very high pressure and temperature conditions. Such a sintering technique is described in Japanese Patent Publication No. 52-12126 (1977), for example. It is known that, among commercially available sintered diamond materials, especially those materials having a grain size not more than several tens of microns have an excellent wear resistance with no cleavage such as that caused in the aforementioned single-crystalline diamond. However, sintered diamond contains several to several 10% of a binder, whereby a cutting edge of such sintered diamond may be chipped due to removal or falling-out of diamond particles forming the sintered body. Since such particle fall-out is remarkably increased as the wedge angle of the cutting edge is reduced, it is difficult to maintain a sharp cutting edge of sintered diamond for a long time. Further, the sintered diamond is inferior in heat resistance to the single-crystalline diamond and easily worn in cutting, due to the binder contained therein.
As compared to sintered diamond, vapor-deposited diamond, which is the second type of polycrystalline diamond applicable to a cutting tool, is superior in heat resistance and wear resistance and is hardly affected by chipping since the same is dense and consists essentially of only diamond. Such vapor-deposited diamond is generally prepared by chemical vapor deposition (CVD) by decomposing and exciting a raw material, which is mainly composed of hydrocarbons such as methane and hydrogen, at a low pressure.
As to a cutting tool employing such vapor-deposited diamond, a diamond-coated tool comprising a tool substrate directly coated with diamond has been developed, for example. In such a diamond-coated tool, adhesion between the tool substrate and a diamond thin film coating the same is important, because the diamond thin film may be separated during cutting work in a tool of cemented carbide coated with such a diamond thin film, for example.
There has also been developed a diamond cutting tool comprising a tool substrate and brazed thereto a member containing vapor-deposited diamond. For example, each of Japanese Patent Laying-Open Nos. 1-153228 (1989) and 1-210201 (1989) discloses a technique of directly brazing a base material, which is coated with vapor-deposited diamond, to a tool substrate of cemented carbide to prepare a cutting tool. In order to apply such a tool to practical use, however, it is necessary to polish the base material or the diamond film on a rake face side. In such vapor-deposited diamond, the diamond surface is so coarse immediately after vapor deposition that it is necessary to polish this surface to use the same as a rake face of a cutting edge. In order to facilitate such finishing, diamond which is vapor-deposited on a base material is generally so formed that a (100) and/or a (110) diamond crystal plane is oriented in parallel with the base material surface. In general, therefore, a rake face of an insert is formed by the (100) or (110) plane. However, such a (100) or (110) plane cannot be regarded as optimum in consideration of wear resistance of the cutting tool, although the same is preferable in view of the aforementioned polishing. Improved wear resistance of the cutting tool can be achieved if the (111) plane is oriented in parallel with the rake face of the tool. However, if the (111) crystal planes of all the crystal grains of a polycrystalline diamond tool material are aligned completely uniformly, then the diamond material, and especially the rake face, will have properties similar to that of a (111) single crystal. Such a rake face has the disadvantage that cleavage and resultant damage to the tool is likely. Therefore, the (111) crystal planes should not be completely uniformly oriented, but rather some fluctuation of the (111) plane is very important to prevent damage caused by cleavage. It is known in the prior art, for example European Patent Application No. 0,319,926 and European Patent Application No. 0,449,571, to synthesize diamond by vapor phase deposition using a material gas having a constant carbon concentration or a continuous or step-wide monotonously varying carbon concentration. However, by following such a method, it is difficult to deposit polycrystalline diamond with oriented (111) crystal planes, with a prescribed fluctuation of the orientation within an appropriate range to avoid the cleavage problems described above.